1. Field of the Invention
The present invention relates to optical communication systems and, more specifically, to monitoring the performance of such systems.
2. Description of the Related Art
Fiber optic networks are widely used for data transmission in modem communication systems. Due to increasing data traffic volumes, monitoring and management of networks become increasingly important. For example, such monitoring may include a measurement at a particular point in the network of the quality of an optical signal corresponding to one or more optical communication channels. The optical signal may be analyzed for various impairments, e.g., timing jitter, chromatic dispersion, and the like. Obtained information may then be used to improve the performance of the network, for example, if certain impairments exceed a tolerable level.
One known method of analyzing an optical signal is to tap that signal, convert it into an electrical signal using an optical-to-electrical (O/E) converter, and then analyze the electrical signal for impairments using electrical signal processing methods. However, one disadvantage of this method is that it typically requires high-speed electronics. Furthermore, at relatively high bit rates, either the sensitivity or bandwidth of the available electronics may be insufficient to accurately and/or cost-effectively measure impairments in the optical signal. Consequently, certain impairments might be missed or certain optical signals may be impractical to monitor by such means. Even when the monitoring electronics has the appropriate bandwidth, monitoring impairments that manifest themselves at amplitudes below the sensitivity of the receiver may be advantageous, for example, (1) in preemptive monitoring, i.e., identifying a problem before it impacts performance, or (2) for monitoring the signal at the transmitter, i.e., under the conditions of relatively high fidelity compared to the signal at the receiver. In optical networks with large regions of transparency, i.e., large distances between O/E conversion points, monitoring becomes particularly important for identifying the location of a faulty component and/or diagnosing the cause of an impairment.
Optical methods applied to monitoring optical signals have certain advantages over electrical methods because at least part of the signal processing is accomplished in the optical domain. This may reduce bandwidth requirements to the monitoring electronics. For example, pulse duration and pulse shape measurements, e.g., to determine optical pulse distortions, may be implemented for short optical pulses using nonlinear optical techniques, such as second harmonic generation (SHG) coupled with relatively slow photodetectors and their corresponding electronics. However, a different nonlinear optical method may need to be applied to measure a different parameter of that optical signal.
Although used in laboratory practice to measure some parameters of optical signals (e.g., nonlinear pulse distortions), nonlinear optical methods are not applied to optical performance monitoring in fiber optic networks. This is largely due to a relatively high cost of nonlinear optical components, performance constraints, and/or system complexity.